Off line Telephone tester         

Here is a circuit of an off-line telephone tester which does not require
any telephone line for testing a telephone instrument. The circuit is so
simple that it can be easily assembled even by a novice having very
little knowledge of electronics. A telephone line may be considered to
be a source of some 50 volts DC with a source impedance of about 1
kilo-ohm. During ringing, in place of DC, an AC voltage of 70 to 80
volts (at 17 to 25 Hz) is present across the telephone line. When the
subscriber lifts the handset, the same is sensed by the telephone
exchange and the ringing AC voltage is disconnected and DC is
reconnected to the line. Lifting of the handset from the telephone
cradle results in shunting of the line's two wires by low impedance of
the telephone instrument. As a result, 50V DC level drops to about 12
volts across the telephone instrument. During conversation, the audio
gets superimposed on this DC voltage. Since any DC supply can be used
for testing a telephone instrument, the same is derived here from AC
mains using step-down transformer X1. Middle point of the transformer's
secondary has been used as common for the two full-wave rectifiers-one
comprising diodes D1 and D2 together with smoothing capacitor C1 and the
other formed by diodes D3 and D4 along with filter capacitor C2. The
former supplies about 12 volts for the telephone instrument through
primary of transformer X2 which thus simulates a source impedance, and a
choke which blocks AC audio signals present in the secondary of
transformer X2. The AF signal available in secondary of X2 is
sufficiently strong to directly drive a 32-ohm headset which is
connected to the circuit through headphone socket SK1 via rotary switch
S2. During ringing, a pulsating DC voltage from transformer X1 via
rectifier diode D5, push-to-on switch S3, and contact `B' of rotary
switch S2 is applied across secondary of transformer X2. The boosted
voltage available across primary of transformer X2 is sufficient to
drive the ringer in the telephone instrument. Please avoid pressing of
switch S3 for more than a few seconds at a time to prevent damage to the
circuit due to high voltage across primary of transformer X2.

The circuit also incorporates a music IC (UM66) whose output is
connected to secondary of transformer X2 via switch S2 after suitably
boosting its output with the help of darlington transistor pair T1 and
T2. This output can be used to test the audio section of any telephone
instrument. After having assembled the circuit satisfactorily, the
following procedure may be followed for testing a telephone instrument:

1. Connect the telephone to the terminals marked `To Telephone Under
Test'and switch on mains (switch S1).

2. To test the ringer portion, flip switch S2 to position `B' and press
S3 for a moment. You should hear the ring in case the ringer circuit of
the telephone under test is working. Please ensure that handset is on
cradle during this test.

3. For testing the audio section, flip switch S1 to position `C' and
connect a headphone to socket SK1. Pick the telephone handset and speak
into its microphone. If audio section is working satisfactorily, you
should be able to hear your speach via the headphone. If you dial a
number, you should be able to hear the pulse clicks or pulse tone in the
headphone, depending on whether the telephone under test is functioning
in pulse or tone mode. If the telephone under test has a built-in
musical hold facility, on pressing the `hold' button you should be able
to hear the music. Now flip switch S2 to position `A'. You should be
able to hear music generated by IC1 through earpiece of the handset of
the telephone under test, indicating propor functioning of the AF
amplifier section. The circuit can be assembled on a small piece of
veroboard. Try to mount the two transformers on opposite sides of the
board, displaced by 90 degrees. Always keep handy multi-type modular
plugs for testing various types of telephones. Mount all switches,
sockets and LEDs on the front of testing panel.